Urethane polymers or polyurethanes (PU's) are a large family of polymers with widely varying properties and uses, all based on the reaction product of an organic isocyanate with compounds containing a hydroxyl group. Polyurethane polymers are generally classified into two broad categories: foam or polyurethane foam, and elastomers or polyurethane elastomers. Polyurethane foams are urethane polymers produced by the reaction of polyisocyanates with a hydroxyl group from a polyol and a polymerization catalyst, in the presence of an auxiliary blowing agent, such as monofluorotrichloromethane or water, which allows the polymeric mass to expand into a cellular mass upon reaction. Polyurethane elastomers are produced by the reaction of an isocyanate with a hydroxyl group to form urethane linkages in the presence of a polymerization catalyst. No blowing agent or mechanism for producing gas which would lead to cell development is present.
Polyurethane elastomers have been widely used in a variety of applications. They have been used as protective coatings, as insulation of electrical elements, as caulks, sealants, gaskets, etc. Because of favorable rheology of an elastomer formulation, they can be used to cast intricate forms such as those found in the toy industry. They have also been widely used in the preparation of sporting goods, fabric coatings and shoe soles wherein the cured urethane elastomer comes in repeated intimate contact with human beings. The prior art catalysts used to prepare elastomers frequently contained toxic mercury and lead compounds and the toxicity was carried over into the cured elastomer. If less toxic organotin compounds are employed as catalysts, elastomers having physical properties less than optimum are obtained.
The production of rigid polyurethane foams is a well-known art, and as such, foams have a wide variety of industrial and commercial applications. Rigid PU foams have been used as packaging materials, flotation materials and various structural components. Rigid PU foam has one of the lowest thermal conductivity ratings of any insulant, which allows efficient retention of heat or, alternatively, maintenance of a refrigerated or frozen environment. Insulating rigid polyurethane foams may be molded into many useful appliances. The foams may be shaped into sheets of varying thickness and placed between roofs or in floors. They also may be formed into contour shapes useful in insulating pipes and ducts. Rigid polyurethane foam can also be applied to numerous substrates by spray foaming techniques. Spray foam applications are important particularly in such areas as warehouses, schools and offices providing the desired insulation requirements for heating and cooling. Polyurethanes are widely used in high resiliency flexible foam seating, rigid foam insulation panels, microcellular foam seals and gaskets, durable elastomeric wheels and tires, automotive suspension bushings, electrical potting compounds, high performance adhesives and sealants, Spandex fibers, seals, gaskets, carpet underlay, and hard plastic parts (such as for electronic instruments).
As a subclass of commercially available polymers, polyurethane elastomers have several properties whose advantages confer unique benefits on these products. Typically, polyurethanes show high abrasion resistance with high load bearing, excellent cut and tear resistance, high hardness, resistance to ozone degradation, yet are pourable and castable. Compared to metals, polyurethanes are lighter in weight, less noisy in use, show better wear and excellent corrosion resistance while being capable of cheap fabrication. Compared to other plastics, polyurethanes are non-brittle, much more resistant to abrasion, and exhibit good elastomeric memory. Polyurethanes can be used for coatings and adhesives, utilizing secondary amine curing agents. Among well-known catalysts used for controlling competing reactions are tertiary amines, organometallic compounds, alkali metal salts of carboxylic acids and carboxylic acids. Also, it may be considered known to cure polyurethane prepolymers with a curing system comprising a dialkyl aromatic secondary amine, a polyol and a primary amine at room temperature using a catalyst system comprising adipic acid and a tin catalyst. However, some applications, such as coatings or repairs, i.e., patches, for concrete structures, such as roads, bridge abutments, parking lots, etc., must also have a very low moisture sensitivity. Therefore, organotin catalysts, such as dibutyl tin dilaurate, or tin(2-ethylhexanoate)oxide, which catalyze the reaction of the prepolymer with water, can only be used in small amounts and care must be taken to keep exposure to moisture at a minimum.
Part of the utility of polyurethanes is derived from their enormous diversity of properties resulting from a relatively limited number of reactants. Typically, polyurethanes are prepared on site by curing urethane prepolymers, which are adducts of polyisocyanates and polyhydric compounds. A large class of such prepolymers are approximately 2:1 adducts of a diisocyanate, OCN—Y—NCO, and a diol, HO—Z—OH, whose resulting structure is OCN—Y—NHCO2—Z—OCONH—Y—NCO. Y can vary greatly, but is usually a divalent alkyl, cyclohexyl, or aromatic radical. In fact the most available urethane prepolymers are made from 2,4-toluenediisocyanate (TDI), or 80/20 mixtures with 2,6-toluenediisocyanate or 4,4′-methylene-diphenyldiisocyanate (MDI). The diols forming the “backbone” of the polymer, containing the “soft segments,” display a greater range of variety. For instance, Z may be a divalent alkyl radical (i.e., an alkylene group) and frequently is an ether or ester which are condensation products of glycols with alkylene oxides and dicarboxylic acids, respectively.
Polyureas are prepared in a similar manner as the polyurethane prepolymers described above except that the backbone of the polymer is formed by the reaction of a polyamine (rather than a polyol) with a diisocyanate. The polyamines and polyols used in the reaction will be referred to as “backbone” polyols or “backbone” polyamines to distinguish them from the curing agents of the present technology.
In the so-called “one-shot” process, a separate step of forming a prepolymer is eliminated and all reactants are brought together at the same time or substantially simultaneously. This term may also be applied where the typical “prepolymer” components are brought together first and within a very short time the curing agent and other additives are mixed together. The “one-shot” method of processing is particularly prevalent in MDI- or modified MDI-based systems. The process generally requires that the various components have similar reactivities with the isocyanate components. The higher heat of reaction creates limitations and some complications with larger cast parts or thicker coatings, but is not particularly deleterious or can be tolerated in the applications contemplated here, and, in fact, may be advantageous in promoting a faster cure without requiring the addition of heat.
Polyurethanes and polyureas are formed by curing the urethane prepolymer. Curing is the reaction of the terminal isocyanate groups of the prepolymer with active hydrogens of a polyfunctional compound so as to form high polymers through chain extension and, in some cases, cross-linking. Diols, especially alkylene diols, are the most common curing agents, especially for MDI-based urethane prepolymers, and representing such diols with the structure HO—X—OH, where X is an organic moiety, most usually an alkylene group, the resulting polymer has as its repeating unit, —Y—NHCO2—Z—OCONH—Y—NHCO2—X—OCONH—, where a triol or a higher polyhydric alcohol is used, cross-linking occurs to afford a nonlinear polymer.
Other polyfunctional chemicals, especially diamines, are suitable as a curing agent. For example, 4,4′-methylene-bis-ortho-chloroaniline, usually referred to as MOCA or MBOCA, is a primary diamine curing agent which is both a chain extender and a cross-linker for TDI-based urethane prepolymers. Generally, however, primary diamines react with prepolymers, and especially MDI-based prepolymers, so quickly that they are not usable as curing agents. Recently, certain secondary diamines have been found to have an acceptably long pot life, and act as chain extenders with urethane prepolymers. Such secondary diamines as N,N′-dialkyl-4,4′-methylene-dianilines, N,N′-dialkyl-phenylene-diamines, and polyfunctional oligomers based thereon, are generally effective curing agents for a broad range of urethane prepolymers at elevated temperatures. Polyhydric alcohols have also been used as curing agents because their reaction with urethane prepolymers is sufficiently fast to be convenient, but not so fast as to make it difficult to work with the resulting polymer. Previous attempts to cure polyurethane and polyurea coatings at ambient temperature have involved the use of a curing agent which includes a primary amine which, as mentioned above, cure very quickly.
Polyurethanes find extensive application as coatings and adhesives. Polyurethanes are particularly desirable because of their chemical resistance, light stability, flexibility, toughness, weatherability, moisture resistance, abrasion resistance, gloss and color retention, and impact resistance. For polymers used in coating or adhesive applications, it is desirable that the tack-free time be reasonably short, i.e., within about 48 hours or preferably within about 18 hours, and gel time long enough for the material to be coated onto a substrate.
Virtually all commercially manufactured polyurethane foams are prepared using at least one catalyst. Catalysts are those compounds that help promote the reaction between an isocyanate and an isocyanate-reactive compound. The types of catalysts that are typically utilized in the formation of rigid polyurethane foams may differ depending on application.
Although organometallic catalysts have found acceptance in many commercial coatings, adhesives, sealants, and elastomers (CASE) applications, their use in urethane-based flexible and semi-flexible foams is limited. Tertiary amines are currently the industry standard polyurethane foam catalyst, but their distinct odor and volatility has caused scientists to search for alternate catalysts.